Robust global output feedback stabilization of underactuated ships on a linear course

This paper develops an output feedback controller that forces underactuated ships to globally ultimately track a straight line under environmental disturbances induced by wave, wind and ocean current. When there are no environmental disturbances, the controller is able to drive the heading angle and cross-tracking error to zero asymptotically. Based on the backstepping technique and some technical lemmas introduced for a nonlinear system with non-vanishing disturbances, an output feedback controller is developed by using a nonlinear observer, which globally exponentially estimates the unmeasured sway and yaw velocities from measured sway displacement and yaw angle. Simulation results on a 32 meters long ship are presented to validate the proposed controller.